This invention relates generally to field effect semiconductor devices having strained heterojunction structures, and more particularly the invention relates to strained heterojunction field effect devices which have improved carrier mobility.
Electrical and optical silicon devices are known which use silicon-germanium alloy superlattice heterostructures and heterojunctions. A major advantage of the Si.sub.1-x Ge.sub.x /Si devices lies in the fabrication compatibility with semiconductor integrated circuit technology. With the advent of molecular beam epitaxy (MBE) and low temperature chemical vapor deposition (CVD) techniques, high quality Si.sub.1-x Ge.sub.x /Si heterostructures or superlattices can be easily obtained. Recently, heterojunction bipolar transistors using the Si.sub.1-x Ge.sub.x /Si strained layers have demonstrated potential applications in high speed circuits. See, for example, Plummer and Taft, U.S. Pat. No. 4,825,269 and Iyer, et al. "Heterojunction Bipolar Transistors Using Si/Ge Alloys" IEEE Transactions on Electron Devices, Vol 36 No. 10, October 1989, pp. 2033-2064. Cutoff frequency of above 75 GHZ has been reported. Furthermore, strained Si.sub.1-x Ge.sub.x /Si quantum well P-channel MOSFETS have been demonstrated using the enhancement and transconductance (g.sub.m) due to the high mobility of the strained silicon-germanium alloy layer. See Nayak et al. "Enhancement Mode Quantum Well Ge.sub.x Si.sub.1-x PMOS" , IEEE Electron Device Letters, Vol. 12 No. 4, April 1991, pp. 154-156. The transconductance (g.sub.m) is shown to be about twice that of the conventional p-doped silicon MOSFET with the same dimension.
A major advantage in using strained Si.sub.1-x Ge.sub.x /Si MOSFETS is improved carrier mobility in the channel by proper control of strain for increasing germanium concentration. However, the mobility enhancement of strained Si/Ge layers for both electrons and holes has a limiting factor. Mobility in semiconductors is in part governed by scatterings due to phonon, impurity or alloy. The dominant scattering mechanism for silicon-germanium alloys lies in the scattering due to the random mixture of silicon and germanium atoms.
The present invention is directed to increasing carrier mobility by reducing scattering in germanium-silicon superlattice structures.